the switch. (Page 76.) For these reasons the singlepole switch is thought suitable only for small tap circuits and its use is limited to branches having on them not more than six 16 candle-power lamps. The making of a connection that completes a circuit, and the breaking of a connection that interrupts a circuit, are spoken of as the "make and break."

Rule 26 (c) is made to provide against the arcing in the switch that would be caused by poor construction.

If the parts of a switch that conduct the current are too small, or if the surfaces that come together do not make sufficient connection, the switch will heat just as a wire that is too small will Resistance is introduced and wherever current

FIG. 63. Jack-knife Switch. heat.

is forced through resistance there is heat.

The "jack-knife switch" is shown in Figure 63. It will be seen that if the switch were mounted so that it would be closed by moving the handle down, it would be closed by anything accidentally falling on it, or possibly by a jarring of the supports. This unintentional closing might cause a short circuit somewhere, as at a motor, for instance. Stops of some kind,

either springs or friction arrangements, are usually provided, but they are not always trustworthy, and it is just as easy and as convenient to mount the switch so that it tends to fall open rather than shut.

27. Fixture Work:

a. In all cases where conductors are concealed within or attached to gas-fixtures the latter must be insulated from the gas-pipe system of the building by means of approved joints. The insulating material used in such joints must be of a substance not affected by gas and that will not shrink or crack by variation in temperature. Insulating-joints with soft rubber in their construction will not be approved.

[Section a. Insulating-joints to be approved must be entirely made of material that will resist the action of illuminating gases and will not give way or soften under the heat of an ordinary gas-flame. They shall be so arranged that a deposit of moisture will not destroy the insulating effect and shall have an insulating resistance of 250,000 ohms between the gas-pipe attachments, and be sufficiently strong to resist the strain they will be liable to in attachment.]

b. Supply-conductors and especially the splices to fixture wires, must be kept clear of the grounded part of gas-pipes, and where shells are used the latter must be constructed in a manner affording sufficient area to allow this requirement.

c. When fixtures are wired outside, the conductors must

be so secured as not to be cut or abraded by the pressure of the fastenings or motion of the fixture.

d. All conductors for fixture-work must have a waterproof insulation that is durable and not easily abraded, and must not in any case be smaller than No. 18 B. & S., No. 20 B. W. G., No. 2 E. S. G.

e. All burrs or fins must be removed before the conductors are drawn into a fixture.

f. The tendency to condensation within the pipes should be guarded against by sealing the upper end of the fixture. g. No combination fixture in which the conductors are concealed in a space less than one-fourth inch between the inside pipe and the outside casing will be approved.

h. Each fixture must be tested for "contacts" between conductors and fixtures, for "short circuits," and for groundconnections before the fixture is connected to its supplyconductors.

i. Ceiling-blocks of fixtures should be made of insulating material; if not, the wires in passing through the plate must be surrounded with hard-rubber tubing.

There are numerous opportunities for trouble in the wiring about gas-fixtures. The fixtures themselves are of metal, and the wires must necessarily be run in confined spaces and close to this metal; the gaspipe system forms a conducting network throughout a building and has good connection with the earth; thus there is not only opportunity for "grounds," but any defect that causes an arc is apt to burn through the gas-pipe, ignite the gas and immediately start a blaze.

The greatest danger of accidental connection is between wires and the fixtures, and consequently it is required that the fixture be joined to the gas-pipe system by means of an "insulating-joint." Figure 64 shows one form. This insulating-joint allows the gas to pass through, but insulates the fixture so that in case a wire becomes electrically connected with it, a connection somewhere else in the building, such

MICA INSULATOR

METAL

B

G C

A

FIG. 64.-Section of FIG. 65. Showing Circuits on Fixtures not proInsulating-joint. vided with Insulating-joints.

as between the other wire of the circuit and another fixture, will not cause a flow of current through the gas-pipe system. Were this to occur there would be a short circuit with a heavy flow of current and there would be formed at the points of contact, an arc that would quickly burn through the pipe. By insulating the fixture entirely, to cause a short circuit it is necessary that two accidental connections on the same

fixture occur, and these two connections must be on

opposite sides of the circuit.

wire, or pole.

one connection for each

Figure 65 illustrates the effect of having no insulating-joints. If, for instance, the wire C coming from the A wire of the circuit were to have connection with the fixture at E, and the wire D coming from the B wire of the circuit were to have connection with any one of the numerous fixtures of the building as at F, then a heavy current would immediately flow from A through C, thence to the fixture and gas-pipe at E, through the gas-pipe system and other fixture to F, through the fixture wire D to B, the other side of the circuit. If there were insulating-joints in these gasfixtures, as in Figure 66, two connections like those supposed could not cause a short circuit, because there would be no path from F to E through the gas-pipe, since the current could not pass the insulation in the joints. To cause trouble it would be necessary to have not only the connection at E, for instance, but another connection with the same fixture somewhere along the wire G. The insulating-joints, by cutting off each fixture by itself, reduce greatly the probability of trouble.

Rule 27 (a) and the accompanying note specify in detail the standard that must be reached by an insulating-joint, and incidently suggest the many ways in